The invention relates to panel form acoustic apparatus using bending wave modes and in particular to loudspeakers incorporating such panels.
Distributed mode acoustic devices are known from copending parent application Ser. No. 08/707,012, which is incorporated herein by reference in its entirety. Such devices do not operate by moving a diaphragm backwards and forwards (pistonically) in the manner of normal loudspeakers. Instead, a transducer is coupled to a stiff panel capable of bending wave oscillations. The bending wave oscillations are distributed over the required frequency range, and couple to the air. The technology is often used in loudspeakers, in which case the transducer is an exciter that excites bending wave oscillations in the panel resulting in an acoustic output.
In WO98/39947, a document that was published after the priority date of the present application, a distributed mode acoustic device is described that can also be operated pistonically. In order to arrange for the centre of mass to be at a suitable exciter location, the distribution of bending stiffness is arranged such that the centre of bending stiffness is offset from the exciter position
It is often advantageous to use a large stiff panel. Large stiff panels give a good high and low frequency performance. However, above the coincidence frequency, at which frequency the speed of propagation of bending waves matches the speed of sound in air, strong beaming can occur.
In smaller panels the effects of coincidence are less of a problem. However, the reduction in size attenuates the low frequency performance and reduces the modal density at lower frequencies, making the response less even.
It is also possible to use a less stiff panel to reduce coincidence effects. This may harm the high frequency performance in two ways. Firstly, the coil mass may have a stronger influence as its impedance becomes comparable to the panel impedance at a lower frequency, affecting high frequency roll-off. Secondly, the aperture resonance of the panel material inside the voice coil, which occurs when the panel wavelength is comparable to the exciter diameter, takes place at a lower frequency for a less stiff panel. This effect can be evident as a peak in the sound pressure. In addition the low frequency performance of a large panel of lower stiffness is relatively poor.
According to the invention there is provided a panel form acoustic member capable of supporting bending wave vibration, wherein the bending wave velocity in the panel is specifically varied in the region of coincidence to produce a range of coincidence frequencies so that acoustic coupling of the bending waves in the panel to the sound waves in ambient air occurs over a broader range of angles and/or so that the acoustical power coupling to the ambient air is more uniform.
The control of coincidence is not a subject of theory or textbook teaching. Although the coincidence effect is known it is treated as a difficulty to be avoided. Alternative methods teach adding mass to the member or coupled layer damping and are generally isotropic treatments.
The panel form acoustic member may be incorporated in any of a number of possible acoustic devices. Accordingly, there may be provided an acoustic absorber, an acoustic resonator for reverberation control, an acoustic enclosure, or a support for audio components including such a panel form acoustic member.
A particularly important application is to a loudspeaker. Accordingly, there may be provided a loudspeaker comprising a panel member capable of supporting bending waves in the audio frequency range, an exciter on the panel member for exciting bending waves in the panel to produce an acoustic output, wherein the bending stiffness of the panel member varies with position over the area of the panel member, so that the effect of coincidence on the acoustic output of the panel is smoothed.
The effects of coincidence on the acoustic output include beaming of sound above the coincidence frequency or discontinuities or peaks in the sound output pressure or power as a function in frequency, integrated over the whole forward hemisphere and/or in particular directions. Using the invention, any or all of these effects may be reduced.
A variation of bending stiffness causes an additional change in the velocity of sound in the panel and hence a variation of the coincidence frequency. The direction of acoustic radiation may accordingly vary over the surface of the panel. The variation of bending stiffness may thus be arranged to cause the distribution of the radiated sound to be spread over a larger angle, to reduce beaming.
Further, in a bending wave panel the power output as a function of frequency often has a peak, step or discontinuity at the coincidence frequency. This irregularity may be smoothed by varying the coincidence frequency.
The coincidence frequency is inversely related to the bending stiffness, and may normally be varied by varying the bending stiffness. This in turn can be achieved by varying the thickness of the panel.
The panel may be stiffer at the exciter location since the aperture resonance caused by a coupling of the coil mass over a finite area is at an advantageously higher frequency for a stiffer panel.
Alternatively, the bending stiffness may have a maximum near the exciter position. For example, the panel can be made symmetric with a maximum in its centre so that the preferred off-centre exciter position for distributed mode panels is close to, but not at, the minimum of coincidence frequency, normally the maximum of bending stiffness. By xe2x80x9cclose toxe2x80x9d is meant sufficiently close that the bending stiffness at the exciter is at least 70% of its maximum; preferably 80% and further preferably 90% higher.
In other embodiments, the panel may be stiffer at the edges of the panel than the centre. The coincidence frequency is still smoothed by the variation in stiffness.
The exciter may be located on the thin region of the panel, where the mechanical impedance of the panel is less. This can aid coupling of lower frequency energy into the panel.
The panel may have a maximum bending stiffness within the central region (the central third both across and along the panel) and reduce in stiffness towards the edges. Such a panel may be formed by injection moulding by gating from the thicker central region of the panel.
The invention may provide the benefits of a large stiff panel whilst reducing some of the disadvantages, in particular the effects of a coincidence frequency within the audio range.
However, the invention is not only applicable to large stiff panels and some good results, presented below, have been obtained on small panels.
In order to have an effect on coincidence, the bending stiffness has to vary over a region of the panel of linear dimension comparable or greater than the wavelength of sound in the frequency range of interest. This may typically be 3 to 4 cm for a frequency of 10 kHz. A very small area of increased bending stiffness is accordingly not suitable to smooth the effects of coincidence. Variation over an area of linear size at least 1.5 times, preferably double the wavelength at coincidence is thus suitable. A variation over an area of at least 5% of the panel area, preferably 10%, may be beneficial for reducing coincidence effects.
Subject to the caveats of the previous paragraph, the bending stiffness variation may be concentrated at the exciter position. For example, the gradient of bending stiffness may be high close to the exciter position and slowly reduce along lines extending outwards from the exciter position. In some embodiments, such a profile gives a useful smoothing of coincidence effects. The gradient can reduce to zero at the edge of the exciter region or the variation can extend to the edge of the panel.
The bending stiffness may be constant in the region of the panel far away from the exciter, with all of the variation of bending stiffness concentrated in the exciter region.
The bending stiffness may also be varied in a strip around the edge of the panel member. The bending stiffness may be maximum at the edge and reduce towards a level in the interior of the panel, or may be a minimum at the edge and increase. Such a panel may have its edge clamped in a frame: the variation of bending stiffness at the edge can then create a desired match or mismatch between the mechanical impedance of the panel and that of the clamping for further control of acoustic output.
The bending stiffness may in particular vary in the edge strip that is no more than a distance of 10% of the length of the panel from the edge.
A reduction in stiffness close to the panel edge reduces the mechanical impedance of the panel in the edge region. If this reduced impedance is less than that of a clamping frame little energy is transferred from the panel to the frame.
Similarly, an increase in peripheral stiffness will increase the mechanical impedance of the panel in that region. If the panel is supported on a resilient support then the increase in panel impedance may create a larger mismatch to minimise unwanted energy transfer to the frame. Conversely, if the panel is connected to a rigid clamp type frame, then this can provide a smoother transition from the panel to the clamped edge and so aid the mechanical robustness of the final construction.
Moreover, in either case a rapidly varying bending stiffness near the edge may reflect acoustic vibration energy back into the interior of the panel so little energy reaches the frame.
The bending stiffness may vary rapidly in the edge region and be relatively constant in the interior of the panel. Alternatively, the bending stiffness may vary over both the edge region and the interior. The bending stiffness may also vary both in the region of the exciter and around the edge, with a region of little or no stiffness variation between the edge and the exciter regions.
Another option is to vary the bending stiffness in an undulating pattern over the panel, or in a plurality of steps.
The coincidence frequency fc, at which the speed of sound in air matches that in the panel, varies as       f    c    =                    c        2                    2        ⁢                  xe2x80x83                ⁢        π              ⁢                  μ        B            
where c is the speed of sound in air, xcexc the areal density of the panel, and B the bending stiffness.
In fact, as well as or instead of varying the bending stiffness any parameter may be varied that changes the velocity in the panel and accordingly the coincidence frequency. Accordingly, it is possible to vary the Young""s modulus of the skin, or the areal density of the skin or core.
In another aspect there may be provided a method of making an acoustical member capable of supporting bending wave vibration, wherein the wave velocity is specifically varied in the region of coincidence to produce a range of coincidence frequencies.
The method may further comprise the steps of selecting a panel material and panel size, selecting an initial bending stiffness profile of the panel, and iteratively varying the panel profile or tensile stiffness of the skin with area to improve the frequency and angle responses of the panel by varying the wave velocity in the panel at around the coincidence frequency to produce the range of coincidence frequencies. In the step of iteratively selecting the panel profile the distribution of resonant modes in the panel over frequency may also be optimised.
In yet another aspect of the invention, there is provided a method of making a loudspeaker system, comprising selecting a panel material, panel size, and exciter type, selecting an initial exciter position on the panel, selecting an initial bending stiffness profile of the panel, iteratively varying the exciter position and panel profile to select a position and profile that optimises the frequency and angle responses of the panel to reduce the effects of coincidence as compared with a flat panel, providing a panel of the iteratively selected panel profile, and affixing an exciter thereto at the iteratively selected position.
The size, profile and exciter position may be selected to produce a distributed mode loudspeaker in which the lower frequency modes are well distributed in frequency and in which the aperture effects are minimised at higher frequencies.